Efficient Degradation of Bisphenol S by Ultraviolet/Persulfate Oxidation in Ultra-Pure and Saline Waters: Effects of Operating Conditions and Reaction Mechanism

As an alternative to bisphenol A, bisphenol S (BPS) is considered an emerging concern. In this study, the degradation of BPS by persulfate (PS), ultraviolet (UV), and UV/PS was comprehensively examined in ultra-pure and saline waters. UV/PS effectively degraded BPS, and the observed first-order rate constant, kobs, increased from 0.021 to 0.382 min−1 with an increasing PS concentration from 100 to 1000 μΜ. The addition of humic acid (HA) inhibited the degradation of BPS, and 1/kobs was directly proportional to the concentration of HA. In salty water containing 540 mM Cl− or 0.8 mM Br−, UV/PS possessed a higher degradation ability for BPS: the corresponding kobs values were 1.45 and 1.66 times that of the control sample, respectively. Eighteen degradation products, including β-scission, sulfate addition, quinone type, ring-opening, and cross-coupling, were identified using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Two possible pathways were proposed: (i) the initial step was considered to be an electron transfer reaction from BPS to SO4•−, leading to the formation of a phenyl radical cation R1, and then phenol radical R4, 4-hydroxybenzenesulfonate cation R5, phenoxyl radical R3, resonant-type carbon-centered radical R2, and their secondary products; (ii) another pathway was the sulfate addition and hydroxylation. These primary reaction sites were further verified by theoretical calculation. This study highlights the effectiveness of UV/PS as a promising strategy for the remediation of BPS and other endocrine-disrupting chemicals in ultra-pure and saline waters (540 mM NaCl or/and 0.8 mM NaBr).